The present invention relates, in general, to a method and apparatus for injecting electrical current into the earth surrounding a well and for detecting and measuring magnetic fields produced by such currents during drilling of the well. More particularly, the invention relates to a system for injecting alternating currents into the earth and for measuring magnetic fields produced by current flow in nearby anomalies such as cased wells, geological formations, and the like to determine the distance and direction to such anomalies, such measurements being accomplished from within a drill string so that removal of the drill string from the well for logging purposes is not required.
In deep well drilling, where wells are commonly drilled to depths in excess of 12,000 ft., it is extremely difficult to track with accuracy the path followed by the drill, since even a very minor deviation in direction can, after several thousand feet of drilling, produce a huge divergence between the actual and intended locations. Because of variations in geologic formations, the high temperatures encountered, and the generally unfavorable conditions which exist in the environment of a deep well, accurate tracking of a well during the course of drilling is extremely difficult. Although numerous techniques have been developed for making such measurements, none have been found to be entirely satisfactory.
Although an accurate determination of well location is always desirable, such information becomes particularly critical in two related situations: first, when an attempt is being made to locate and intercept a well, and second, when an attempt is being made to avoid another well known to be located in the general area being drilled. The first situation may occur, for example, when an existing well blows out and it becomes necessary to drill a relief well that will intersect the first, or target, well at a location below the blowout. The relief well must be started a long distance from the target well head, for example as much as two miles away, and must angle down to intersect the target well at a depth of, for example, 10,000 to 15,000 feet. Although the initial part of the drilling can be done using existing directional equipment, a problem arises when the relief well nears the target well, since accurate information about the relative locations of the two wells has not been available in the prior art. Not only must the measurements be made at locations which are thousands of feet below the surface of the earth, but in addition the same problems of tracking existed when the target well was drilled as when the relief well is being drilled. As a result, only a slight deviation from the assumed location and direction of the relief well can result in premature intersection with the target well or can cause the relief well to miss the target well entirely.
The second situation noted above occurs, for example, when several wells are being drilled from a single location, as when a large number of wells are being drilled from an off-shore platform, and later wells must be directed so as to avoid earlier-drilled wells. More particularly, the problem arises when it is desired to add a well between existing wells while avoiding intersecting the existing wells. This occurs, for example, were some of the early drilling from off-shore drilling platforms located well heads at 12 foot intervals, with the wells themselves being directed away from each other in various directions. In order to increase the oil production from such a drilling platform, it may become desirable to drill new wells between the existing wells; that is, at 6 foot intervals, so as to increase the number of wells served by the platform. However, it is essential that the new wells avoid the existing wells, and great care must be taken to monitor the direction of drilling.
Another problem which is encountered in deep well drilling is the unexpected crossing of the interface between different geological strata into a stratum which may contain, for example, gases at very high pressure which can damage the drilling equipment. In order to avoid such occurrences and in order to determine the nature of the strata through the drill is travelling, periodic logging of the well is required.
It is known that the magnetic and electrical characteristics of the geological formations surrounding a well being drilled, which characteristics reflect the presence of anomalies such as variations in the strata or the existence of a target well casing, can be measured by means of highly sensitive magnetometer systems. In such systems, for example, electrical currents are caused to flow in the strata surrounding the well, the current flow creating a magnetic field that can be measured. The injected currents do not flow uniformly through the strata, but may be concentrated in conductive anomalies such as the metal casing of a target well, thereby creating a detectable variation in the magnetic field. Such a system is disclosed in U.S. application Ser. No. 203,912, filed Nov. 4, 1980, now U.S. Pat. No. 4,372,398 entitled "Method of Determing the Location of a Deep-Well Casing by Magnetic Field Sensing", of Arthur F. Kuckes. This application discloses a method for injecting current and measuring the resultant magnetic field utilizing a highly sensitive magnetic field sensor such as that shown in U.S. Pat. No. 4,323,848, issued April 6, 1982.
Although such prior systems have been effective, a difficulty has been encountered because of the fact that they cannot provide measurement during the actual drilling of the relief well. Such systems require the periodic removal of the drill string and insertion of the measuring equipment to allow logging measurement to be made. Because of the extreme depth of such wells, the removal of a drill string is extremely expensive, and results in a large amount of lost drilling time. Such lost time is particularly critical when an attempt is being made to intercept and shut down a blown out well. Furthermore, on occasion the logging equipment itself becomes stuck in the relief well, further delaying drilling and increasing costs while attempts are made to free it. Since logging measurements must be made more and more often as the relief well approaches the target well the costs and time delays involved become considerable.